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Scientists expect an explosion of lemmings in southern Norway this summer, after a mild winter allowed the arctic rodents to breed rapidly under the snow.

Nina Eide, a senior researcher at the Norwegian Institute for Nature Research (NINA), told The Local that her institute was expecting a rapid rise in lemming numbers in southern Norway -- a so-called 'lemming year'.

"It seems that it will be a peak year in several parts of southern Norway," she said. "They're already on top of the snow, so I think they will peak this summer, or maybe even a bit earlier. You will have a huge number now in spring and then in early summer, we will see a sudden crash."

Rolf Anker Ims, a biology professor at the University of Tromsø, told The Local that far northern Norway would see a lean year for lemmings, with his research indicating that many lemmings had been killed off during the winter.

"In the northernmost part of Norway, the country of Finnmark, the situation seems to be rather bad for the lemmings on the grounds that the snowpack seems to be rather hard close to the ground," he said.

His research now indicates that the quality of snow cover is the most important of determinant of whether lemmings emerge in large numbers in the summer, with 'lemming years' occurring when there is thick snow cover but a mild enough temperatures for pockets to form where the animals can feast on moss beneath the snow and continue breeding.

"We have discovered that the reason lemmings can suddenly appear in large numbers is because they are able to increase in numbers through the winter," he said. "They can breed under the snow, and this is something mice do not usually do."

He said the snow pits he had dug in Finnmark showed that winter rainfall had penetrated right down to the rock, where it had frozen solid, making it hard for lemmings to survive.

He argued that the number of lemmings spotted out on the snow in southern Norway, which Eide interpreted as a sign of a large population surviving the winter, could equally mean the animals are in trouble.

"That might mean massive amounts of lemmings, or it might mean that the snow conditions are so bad, that they're digging themselves out of the snow pack," he said. "That's a sign that the population is already crashing."

According to Eide, the instability of lemming populations follows from their extremely rapid reproduction. "You get mothers and grandmothers and great grandmothers all in one season," she said. "They are born and then three weeks later they can get pregnant."

This means that when conditions are good, populations explode, with massive groups of lemmings stripping whole areas of grass-level vegetation before the population crashes suddenly.

The sight of large groups of the rodents migrating together during lemming years has given rise to the popular misconception that the animals commit mass suicide when their numbers reach unsustainable levels.

In fact, Eide said that the reason for the sudden crashes in lemming populations was still debated by scientists.

"We really don't know why they crash if there's a high population density," she said. "It's a mystery why they crash so suddenly or where they go."

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